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Ian Mackenzie
09-13-2003, 02:43 AM
Can anyone researched or compared the acoustic qualities of these materials for baffle construction?

Ian

Alex Lancaster
09-13-2003, 08:14 AM
JBL pro, 20+ years ago, distributed a white paper discussing diff materials, I believe by John Eargle; I cannot find it, maybe smeone remembers it?

Alex.

Tom Loizeaux
09-13-2003, 09:21 AM
The common understanding is that MDF is less resonate than the equivalent thickness plywood. This is due largely to the fact that MDF is made from "broken up" wood fibers, pressed and mixed with glue. This seems to deaden the material where plywood, because of the continuous structure, resonates a bit more.
My concern about MDF is that it is more suseptible to breaking, chipping and moisture damage.
My choice in building speaker cabinets would be to combine these two materials by building the box with MDF but covering or laminating the outside with quality, thin-ply plywood, trying to use at least 1/4" material on the outside.

Tom

DavidF
09-13-2003, 10:44 AM
MDF is more dense than either plywood or particle board (“PB”). The particles worked into PB are larger milling pieces that create too many voids, a common problem with plywood. MDF material is ground-down to a finer base composition. MDF therefore is more mass per square inch, works better with cutting tools, and is usually easy to locate. But as Tom mentions, it has serious drawbacks as being fragile until finished and makes very fine dust in working it. It is not suitable as a base for a mirror/gloss finish because it is too easily “bruised” while working the piece with tools. Belt sanders are a bad idea. Sharp bits and blades are a must which, unfortunately, will dull quickly with MDF. If you can keep it clean from such marks, and NOT DROP IT-EVER, and get some hard finish to protect it, it work-out much better than PB.

David F

Mr. Widget
09-13-2003, 11:19 AM
MDF is distinctly superior acoustically and slightly inferior structurally to particle board. When MDF first became available in my area in the late 70's I built identical cabinets out of both particle board and MDF. The MDF was cleaner sounding in the low mids and the bass was deeper and tighter as well.

Birch multi-ply from the baltic countries is the superior structural material, but even if you brace the hell out of it, it is not as acoustically inert as MDF.

I agree with Tom's recommendation to laminate or skin the MDF box with plywood. This is only necessary if you plan on moving the speakers around.

For my 4355 project I will be using this technique with 3/4" plywood and 3/4" MDF as I will need to wheel the beasts out of the center of my HT when I want to watch a movie.

Ian Mackenzie
09-13-2003, 08:31 PM
Hi,

Thankyou all for your informative responses.

I will certainly look at the sandwich panel approach.

Maybe two 3/4 inch MDF panels laminated (or a MDF & PB) with PVA glue would be a good compromise and a hardwood frame around the edge of the baffle so I can use the router for a tapered moulding.

Not sure yet.

Yesterday I had a practise with the plung Router (Portor Cable) & Jasper jig using 3/4 inch particle board.

The circle was indeed smooth and accurate

regards

Ian:)

Ian Mackenzie
09-13-2003, 10:11 PM
Just had a look at the JBL Consumer site, and figured this stuff may appeal to others cloning the 4345's so here it is:

JBL appear to be using MDF in the new k2 series and laminates of MDF in some cases so I suppose that is a good indication of MDF's sonic qualities.

On reading the JBL 2 literature, reference is also made to use of small baffle surfaces and multi shaped cabinets to minimise vibrations and reflections. Well, that's a bit of a problem with an 18 inch woofer....hum bug.......

They also refer to vibration transfer to the floor using careful cabinet design. The resonance-free enclosure is crafted of highly rigid, heavily braced, 1-inch (25.4mm) MDF panels, and the woofer baffle is further reinforced with a 0.75-inch (19mm) MDF panel. An irregular hexagonal prism structure increases rigidity and keeps panel sizes to a minimum while the cabinet base comprises layers of MDF and birch primary board. Four bolts fix the base firmly to the bottom of the enclosure.

Cool design.......
http://www.jbl.com/home/k2_story/k2_tech1.asp#enclosures


Some DIY research......

I read in a couple of books (by Martin Colloms - Hi Performance Loudspeakers & Great Sound Stereo Speaker Manual by David B . Weems & G.R Koonce) last night and there is considerable debate about the best practise for enclosure panels.

In general PB and MDF are the preferred materials but there are conflicting views regards density. Apparently it takes more energy to move a high density wall, but the greater the density and mass, the slower the energy is released again. This stretching of pulses can blurr transients. (~golly, I never it was so technical).

Reference is also made to thickness of materials and 3/4 inch is a good choice, while some builders use combinations of differing materials on the theory that one can damp the other. Others use multiple layers of a single material to make a thicker board, particularly the speaker board.

Recent experience suggests such practises can introduce more problems than they solve (~Oh no). References are made to imperfect adhesion of between the panels. Further studies suggest 3/4 to 1 inch materal may have the optimum thickness for enclosures. Thicker material can result in the excess density problem mentioned earlier.

The use of damping pads and very dense materials like pure lead are also used for enclosure panel damping.

So perhaps my best option as I am retrofitting a new baffle might be to leave the exisiting baffle in place around the woofer area for strength if possible and cut out the remainder of the exisitng baffle leaving a 2 inch lip around the outside. Then carefully laminate and scew down a 1 inch MDF panel.

If I am feeling ambitious and have a few spare dollars I may invest in some lead sheeting for some of the broader panel areas in the hope that it will create a vibration fee enclosure.

(I think I will just make it and see that happens...muhhahaha)

Thankyou again for your practical hints (experience counts!)

Ian

sonofagun
09-14-2003, 11:02 AM
With my background and experience in speaker building here are some thoughts on this very interesting topic:

Virtually all materials commonly used (and useful) for speaker cabinet construction will resonate to one degree or another. Once energy (acoustic and otherwise) is created, it cannot be destroyed (or eliminated); only transferred to another place or form.

There are many types of wood based panel products that have been used at one time or another for cabinet construction. A list includes:

1. Particle Board
2. So called "MDF" (medium density fiberboard)
3. Plwood
4. Solid wood

There are numerous sub-types within some of these groups:

#1 - low density (unusable) and high density (cabinet grade-desirable).
#3 - many grades from low grade construction board no good for speaker cabinets to high grade, multi-ply, void free board (very 'spensive!).
#4 - numerous species, grades, and planking (but really rarely used due to cost and warpage/stability problems).

The resonant quality of any product is determined by it's physical and molecular composition. Traditionally resonances have been ameliorated by one or a combination of minimal panel size(s), cabinet design, bracing, acoustic dampening material linings, sandwich construction, and/or greater panel thicknesses.

Interestingly, I just performed a simple experiment - went out to my garage and suspended a plank of MDF and tapped it with a hammer. The result - I could hear a specific "tone" or frequency. Tried the same with various size planks (all 3/4") of pine board, and particle board (cabinet grade) and they all resonate at a specific freg. In my opinion, MDF is more resonant than PB due to its finer grained (and therefore less air content) composition.

Perhaps safe to say here that some materials with very LOW densities (and therefore high air content) are the least resonant. Examples are styrofoam board, fiberglass insulation, and acoustic poly-foam commonly used as lining material.

Supposedly the ultimate material is concrete as most of you are aware, but rather impractical! HOWEVER, I'm sure if the sound is loud enough, you can hear it through concrete walls (as some irate apartment dwellers can attest!) :biting:

Somewheres I may still have an old book on building enclosures (forget the title) written by the same people (apparently) who ran Wharfdale speakers. Was a chapter or two on panel choices and resonances. Of course, they pioneered the sand filled cabinet wall constuction.

MDF is very weak structurally to work with - holds screws well only crosswise to panel. Does cut VERY easily with carbide blades as long as you clean the residue off the teeth regularly (applies to cutting all wood products with carbide).

Secret to successfully joining fiber/wood based products is proper use of the wonderful glues we have available. When joining pieces, you need to first "prime" them using a slightly thinned down amount of the adhesive you're using (most are water based such as "titebond") and allow it to soak into the substrate for a while and later apply the full strength glue, assemble and clamp in place. I'm rather against using screws as properly assembled glue joints are stronger than the wood itself!

To quote from "How to Build Speaker Enclosures" by Badmaieff and Davis:

"Brick, stone, cement, etc., make suitable rigid materials, but are, of course, more difficult to work with and restrict mobility. Some constructors in years gone by have made enclosures with double walls and filled the space between with sand. All of these ideas are intriguing, but as has been steadily pointed out in this book, good engineering results in a balanced approach to the problem, and 3/4-inch plywood represents, if properly handled, the very best results for the investment involved."

Just a few thoughts for starters.

Mr. Widget
09-14-2003, 12:43 PM
Nice theory son...:D

But seriously, you are right that every material will resonate. Concrete can ring like a bell. I had some sonotube sub cabinets for a while that were dual walled (concentric) sonotubes with the gap between them filled with sand. Since the walls were not parallel and the sand filling made them very dense without concrete's ring they were about ideal for a sub cabinet, but they were almost impossible to move, and I didn't want big cylinders in my room- even if they were veneered in rosewood!

From a practical standpoint MDF just sounds better than most other sheet woods. Solid wood construction has it's proponents, but it has more than it's share of problems too. JBL used particle board back in the 70's as did most everyone else, by the 80's most manufacturers were using MDF and eventually JBL moved on to MDF too. Since MDF costs about twice as much as particle board, and since few consumers pay attention to core material I doubt they made the change for marketing reasons. Short of going to the trouble of sand filled cabinets MDF is the most practical solution. But hey for the DIY crowd, sand is cheap, if you want the ultimate and don't mind the hassle, summer is over, maybe one last trip to the beach is in order:smthsail:

Ian Mackenzie
09-14-2003, 04:11 PM
Hey Man,

My beach is only 5 minute walk, time for the bucket and spade.

On a more practical note, the sans needs to be dried and panel design is more complex.

With the big boxes using multiple drivers we have I think this aspect of box colouration is not overly critical.

Because we have multipe drivers using different frequency bands and sub enclosures I think as long as the woofer box is rigid with bracing and does not resonate in the intended band and likewise for the mid box the sound should remain crystal clear.

Smaller designs incl 2 ways with crossover points of 2K would be a more difficult problemas you have active midrange bouncing around inside the box. In most cases out woofers are crosse over 250-300 hertz, then to a smaller rigid sub enclosure.

Ian

sonofagun
09-14-2003, 06:35 PM
Originally posted by Mr. Widget
Nice theory son...:D



I'm sorry but I'm not sure what you mean. Please explain - am I in error on something? Been a long time since I was building enclosures AND I am a newcomer to these forums - I was just trying to be helpful.

Mr. Widget
09-14-2003, 08:56 PM
Hey I was just giving you a little ribbing. Basically I was agreeing with most of what you said.:)

sonofagun
09-15-2003, 08:00 AM
Ribbing? Ribbing? What's:rotfl: tha:rotfl: t?
Not sure:spin: what you:rotfl: :rotfl: :rotfl:?

Alex Lancaster
09-15-2003, 11:55 PM
As posted before, I had this 4-5 page paper sent by JBL to pro dealers, discussing diff materials; I´ve looked for it, but could not find it.

Anyway, I remember that a laminate made from slats and plywood was recommended, it even went into diff woods and more things I do not recall (20+ years).

If anybody knows about it pls let us know.

Thanks, Alex.

Ian Mackenzie
09-16-2003, 04:08 AM
Thanks Alex,

Ian